This invention relates to a method for enhancing signals transmitted over fiber optic wave-guides, generally known as optical fibers. It specifically is directed to enhancement of short, spectrally broadband optic pulses transmitted through an optical fiber having substantial length.
The signal enhancement technique which is the subject of this disclosure arose from developments relating to optical measurements in high frequency plasma diagnostics. It provides prompt diagnostic signals derived from fast radiation pulses such as produced in connection with the detonation of nuclear explosions. In such applications, a radiation-to-light converter could be an optical fiber or a red fluor placed in the plasma radiation beam as a Cerenkov transducer or fluor transducer, respectively. U.S. Pat. No. 3,984,322 to Melvin A. Nelson, Terence J. Davies and John R. Morton, III, assignors to the assignee of the present application, is directed to a radiation detection system utilizing optical fibers as Cerenkov transducers and U.S. Pat. application Ser. No. 949,163, filed Oct. 6, 1978 in the names of Larry A. Franks, Stephen S. Lutz and Peter B. Lyons, also assignors to the assignee of the present application is directed to a radiation detection system utilizing optical fibers in combination with fluor transducers. The signal source might also be a Light Emitting Diode (LED) driven by an electrical signal derived from the radiation. The signal enhancement technique according to the invention is also applicable to the communications industry. It would enable relatively inexpensive spectrally broadband Light Emitting Diodes to be used in transmitters for long high-bandwidth optical fiber links in place of relatively more expensive lasers.
Broadband signals transmitted through optical fibers are degraded by material dispersion. Conventional methods used to reconstruct such signals and preserve their high frequency components involve initial selection of an appropriate limited bandwidth by use of a narrow band filter or monochromator. The filtered signal transmitted through the optical fiber is then detected by use of a high speed detector. The resulting electrical signal is subsequently processed. This approach has two disadvantages: first, narrow band filters have "wings" that degrade the system frequency response and, second, most of the available signal is undetected.
The present disclosure replaces the narrow band filter with a fiber optic wavelength multiplexer device. The broadband signal input is initially dispersed and narrow spectral components are collected into an array of fibers. Each fiber is used as an optical delay line to compensate for the material dispersion of its spectral component during the original signal transmission. These equalizing fibers, each cut to an appropriate length, couple the spectrally equalized light onto a detector for further processing.
This technique increases signal magnitude by utilizing more of the available broadband spectrum than is possible when using narrow band filters. It also improves frequency response by eliminating the common "wing" contributions of such filters.